1. Field of the Invention
The present invention relates to a color-corrected and temperature-compensated lens which can correct chromatic aberration and compensate for defocusing upon temperature at the same time and, in particular, to an image readout lens used for reading out an original with a color image or the like.
2. Description of the Prior Art
There has been widely known in general that secondary spectra of chromatic aberration can effectively be corrected by means of a lens made of a material having an anomalous dispersion characteristic.
For example, a readout lens with a seven-sheet configuration such as that shown in FIG. 28 in which a concave lens L.sub.3 and a convex lens L.sub.4, which are respectively the third and fourth lenses from the object side, are cemented together to form an achromatic lens can attain a relatively large common depth of 0.37 mm, for example, with respect to three color light components of R, G, and B, thus allowing the secondary spectrum of chromatic aberration to be effectively corrected.
On the other hand, materials with an anomalous dispersion characteristic typically have a large coefficient of linear expansion .alpha., and its change in refractive index with respect to change in temperature, dn/dt, is a large negative value.
Accordingly, a coefficient .chi. concerning temperature inherent in glass represented by the following equation: EQU .chi.=.alpha.-dn/dt.multidot.[1/(n-1)]
becomes a large value.
Since the amount of movement of focus upon change in temperature depends on the refracting power .phi. and the temperature coefficient .chi., the amount of focus movement with respect to change in temperature increases as the temperature coefficient .chi. is greater.
As the prior art for correcting the movement of focus with respect to change in temperature, it has been known to combine lens-forming materials under a certain condition so as to suppress the amount of focus movement to a certain extent as disclosed in Japanese Unexamined Patent Publication No. 4-298709.
This prior art, however, does not take much account of the correction of chromatic aberration and, in particular, does not consider any correction of the secondary spectrum of chromatic aberration.